This disclosure pertains to an improved process for hydroformylating an olefinically-unsaturated compound to produce one or more aldehyde products.
It is well known in the art that one or more aldehyde products can be produced by contacting under reaction conditions an olefinically-unsaturated compound with carbon monoxide and hydrogen in the presence of a metal-organophosphorus ligand complex catalyst. One such process, as exemplified in U.S. Pat. Nos. 4,148,830, 717,775, and 4,769,498, involves continuous hydroformylation with recycle of a solution containing the metal-organophosphorus ligand complex catalyst, more preferably, a Group VIII-organophosphorus ligand complex catalyst. Rhodium is a preferred Group VIII metal. Organophosphines and organopolyphosphites are preferred organophosphorus ligands. Aldehydes produced by hydroformylation processes have a wide range of utility, for example, as intermediates for hydrogenation to aliphatic alcohols, for amination to aliphatic amines, for oxidation to aliphatic acids, and for aldol condensation to produce plasticizers.
While the benefits attendant with rhodium-organopolyphosphite ligand complexes for hydroformylation processes are well known, the stability of the organophosphite ligand remains a primary concern. Degradation of the organophosphite ligand can lead to catalyst poisons, inhibitors, or acidic byproducts that can lower catalyst activity or increase the rate of ligand loss. Because rhodium is extremely expensive, significant loss of catalytic activity can have a dramatic impact on process economics. Moreover, organopolyphosphite ligands are manufactured through multi-step syntheses, and are often quite expensive in their own right. In order for a rhodium-organopolyphosphite-based industrial hydroformylation process to be economically feasible, the ligand must be stabilized against the rigors of process conditions.
Numerous methods have been proposed to maintain catalyst and/or organophosphite stability through the addition of a second phosphorous-based compound. For instance, U.S. Pat. No. 6,153,800 describes adding sterically-hindered phosphines (e.g. tri(ortho-tolyl)phosphine) to stabilize a phosphite ligand against oxidative degradation. While less costly than the organophosphite ligand, the addition of tri(ortho-tolyl)phosphine) would significantly impact industrial process economics. Although less expensive, less sterically-hindered phosphines (e.g. triphenylphosphine; hereafter TPP) are effective in protecting the organophosphite, U.S. Pat. No. 6,153,800 teaches that they also significantly decrease the hydroformylation rate of the catalyst.
Organophosphite ligand stabilization is discussed in CN 1986055 A, which features the addition of tri(aryl)phosphines (e.g. TPP) to a rhodium/bisphosphite system. Although the bisphosphite ligand/TPP combination is more stabile than the bisphosphite alone, the hydroformylation reaction rate is significantly reduced. The loss in reaction rate can be recaptured by increasing the reaction temperature, but higher temperatures are known to increase the formation of aldehyde oligomer “heavies,” which lowers the overall reaction efficiency.
Accordingly, it would be desirable to have a hydroformylation process employ a relatively inexpensive method for the stabilization of a rhodium/organophosphite ligand system that would not significantly reduce the inherent hydroformylation reaction rate of the system.